Retinitis pigmentosa (RP), one of the most frequently occurring blinding retinal dystrophies, is characterized functionally not only by nightblindness and peripheral visual field defects, but also by impaired foveal vision. The University of Illinois Research Center of the National Retinitis Pigmentosa Foundation Fighting Blindness has a cohort of approximately 900 well-categorized patients with RP who actively and frequently participate in basic and clinical research projects. Properly posed psychophysical tests on carefully selected individuals and subgroups of patients from this cohort provide a unique opportunity to assess the nature of the underlying pathophysiologic defects in retinal dystrophy, as well as to determine basic neural mechanisms for normal visual function. The results of such studies will also lead to the earlier detection of vision loss in RP, to more precise characterization of visual impairment in individual patients, to optimization of visual rehabilitation (low vision aids), and to more sensitive evaluation of potential therapeutic regimens. The goals of this research project are to test specific hypotheses concerning retinal functioning RP. Specifically, the project will determine the impact of RP on spatiotemporal mechanisms of vision and the ability of patients with particular subtypes of RP to process spatial and temporal visual information. In doing so, the research will also further clarify the extent and underlying mechanisms of foveal impairment in RP. The specific aims are: (1) to assess the contribution of irregularity in receptor sampling to foveal dysfunction in RP, (2) to evaluate the significance of contrast polarity and temporal parameters in the measurement of contrast sensitivity for letter identification, (3) to ascertain the relationship between detection of spatially localized, frequency band-limited visual stimuli and the complex task of letter identification, (4) to define the effect of light adaptation on properties of spatial mechanisms, and (5) to dissect the relative contributions of abnormalities in spatial vision and adaptation to temporal sensitivity losses in RP patients.